In recent years, portable electronic devices, such as cellular telephones and personal digital assistants, have become commonplace. Many of these devices include a Radio Frequency (RF) modulator section. The RF modulator can reside within a transmitter unit on a portable device to modulate base-band signals to communication signal frequencies for transmission whereby the communication signals are broadcast to other portable units with a RF modulator at the receiver unit capable of demodulating the signals back down to base-band. The base-band signals can be decoded into an audio signal and broadcast through a speaker to a user of the receiving portable electronic device.
Many of the portable handset devices include a high-audio speaker to play the audio signal at higher volume levels. A power amplifier is generally coupled to the speaker to amplify the signal sufficiently such that the user can adequately hear the output audio. The high audio speaker is generally a transducer which converts electrical signals to mechanical movements through the electro-magnetic coupling of a permanent magnet and voice coil attached to a diaphragm. The movement of the diaphragm moves air and thereby creates pressure differences which produce an acoustic signal.
The speaker needs to move a large amount of air to produce a high volume audio signal where the pressure level is proportional to acceleration of the air. Accordingly, a large amount of force is required to move the air at the diaphragm where the amount of force is a function of the size of the diaphragm and the size of the magnet. The forceful movement of the diaphragm at high audio levels can also push air into and out of the handset creating pressure which accordingly produces vibrations in the handset device. Also, when the handset is not properly enclosed or sealed, the internal acoustic pressure can leak to other compartments within the handset. The problem is exacerbated when the speaker is in close proximity to the electrical board components. All devices and components internal to the handset can be subject to these vibrations. These vibrations can induce bending of component boards such as those that house the RF modulation circuitry.
The electro-mechanical-acoustical stress and strain bending of the boards can change the electrical properties of the integrated circuits which can in turn alter the behavior properties of the device. For an RF component such as a Voltage Control Oscillator (VCO), the mechanical bending can vary the voltage, and, the VCO frequency deviates in relation to the bending. The deviation effectively superimposes properties of the acoustic signal onto the demodulated signal. In effect, the physical bending can modulate the behavior of the demodulator where the result can be regeneration of the output audio on top of the demodulated signal. This behavior is a feedback loop which can oscillate and go unstable when the signals become highly correlated, or in phase. In effect, the regenerative audio feedback acts as a parasitic modulation that gets demodulated and amplified over and over causing oscillatory feedback, commonly called ‘microphonics’. The internal pressure is inversely proportional to the internal air volume. And, as handsets become smaller the microphonics problem can continue to increase. Accordingly, a smaller handset can go unstable at high volumes which causes a howling effect as a result of receiver audio regeneration.
Solutions to avoid the bending of the circuit boards include material padding to absorb the sound, mechanical ribs or clips to limit the allowable degree of mechanical bending, and non-piezoelectric capacitors. The current approaches attempt to minimize the acoustic pressure build-up and/or isolate the acoustic coupling. They rely on mechanical solutions that can not fully resolve the howling problem caused by the regenerative audio feedback. In addition, system engineers set a specification margin for certain parameters in shipping radios to account for tolerances in parts and variances in temperature. However, this lowers the overall volume gain of the handset. A final recourse, when the mechanical solutions are insufficiently capable of mitigating the howling behavior, is to lower the level of high audio speaker output by setting a maximum volume level corresponding to a gain specification level below which howling occurs. Accordingly, the handset is shipped with a reduced loudness gain to meet the gain specification margin. However, this reduces the overall loudness level which users expect from a high audio speaker handset. In a public safety environment, or other high ambient noise condition, such restriction may not be acceptable.